Q: Inertial Frames & Detecting Motion

[mjacobsca]

In a true inertial frame inside a train compartment, it is impossible to do an experiment that can tell you if you are moving or not. This is a key consequence of Newton's Laws Of Motion. However, if someone had a neutrino detector, or a Higgs detector (assuming they exist), wouldn't it be impossible to have a pure inertial frame of reference, ever, since we could always determine motion? Would this have any ramifications for special relativity or Newton's Laws Of Motion? Or, as I expect, does the train car analogy assume that nothing, not even a hypothetical bit of space-time foamy quantization, could be detected from inside the train car?

 

[JesseM]

In a true inertial frame inside a train compartment, it is impossible to do an experiment that can tell you if you are moving or not. This is a key consequence of Newton's Laws Of Motion. However, if someone had a neutrino detector, or a Higgs detector (assuming they exist), wouldn't it be impossible to have a pure inertial frame of reference, ever, since we could always determine motion? Would this have any ramifications for special relativity or Newton's Laws Of Motion? Or, as I expect, does the train car analogy assume that nothing, not even a hypothetical bit of space-time foamy quantization, could be detected from inside the train car?

Higgs particles and neutrinos do not all have a common rest frame (nor is the space-time foam expected to), if these particles were created during the course of the experiment by the lab apparatus then they should behave the same (on average) regardless of the train's state of motion. If you're talking about cosmological neutrinos, the principle of relativity just says the laws of physics work the same way in all frames, so if you start with the same initial conditions of particles in a train, they will behave (on average) the same way subsequently regardless of the state of motion. Cosmological neutrinos coming from the outside might look different depending on the train's speed relative to the CMBR, but this doesn't change the fact about the laws of physics being the same.